Firstly, when you entered the bill for the truck, you SHOULD have posted it to a Fixed Asset account called Truck which would be the value of the truck on the bill, less any HST or other expenses (eg. fuel tax). Once purchasing the vehicle you owe the Vendor for the vehicle.
THEN, you post a G/L transaction for the loan, paying the vendor and establishing the loan payable. Post a Debit to Accounts Payable for the amount owing to the dealer, and post the Credit entry to Truck Loan Payable, your Long Term Liability.
Do you know the amount of interest you will be paying over the course of the term of the financing? Your interest rate should be on your loan documents. If not, it should be the difference between the cash value of the vehicle (bill from vendor) and the total of all the payments you will be making over the term of the loan. EG. truck costs 18,500, HST totals 2,775. Total cost (including freight, delivery, fuel tax, etc.) 21,275. Loan payments are, say, 675.00 per month for 36 months, totalling 24,300. Therefore, interest totals 24,300 minus 21,275 = 3,025.
You then need to establish the liability for Loan Interest Payable. Post a G/L transaction Debiting Interest Expense and Crediting Loan Interest . YES, Interest Expense. You signed the loan agreement at the BEGINNING, therefore you are legally obligated to pay both the interest and the principal, establishing the debt for BOTH.
As you make your payments, you write a cheque and post entries to the Loan and Interest Payable account. If you have an amortization schedule from the financing company, use that to determine how much the principal and interest payments are monthly. If not, you can do 1 of 2 things: use a straight line method, not recommended in these days of compound interest. The other is to do up a simple Excel spreadsheet to calculate interest each month, but you need to determine if the interest is calculated BEFORE or AFTER the payment is made. This WILL make a difference.
I know this seems overly complicated, but believe me, when your accountant gets a look at the results, you will have fewer phone calls and hassles.
PLEASE see a professional accountant and tax advisor.