You would have to examine the air returns and heat vents for each furnace; if a return or vent is blocked with the slides in , then there is a chance you could overheat the furnace due to the restricted air flow. If no restrictions I'd say your good to use them.
If your still in doubt, maybe a call to Tiffin is in order.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.